The invention pertains to a process and to a system for the production of plates of quartz glass.
It is known that, to obtain plate material, slices can be sawed off from quartz glass blocks, which are very expensive to produce primarily because of the large amount of energy which must be consumed. After a post-treatment of the cut surfaces, the plates thus produced meet stringent requirements with respect to surface quality and flatness. The cutting tools used for sawing, however, are very expensive. The production of the blocks and the sawing operation itself are extremely time-consuming and associated with the loss of materials. The thinner the plates sawed from the blocks, the greater the loss of material.
A process for the production of quartz glass plates is known from GB-A 2,184,434, in which a quartz glass tube is slit open in the longitudinal direction and then softened to form it into a plate. The plates thus produced, however, are not flat. Small depressions and elevations remain after the shaping process.
Another process for the production of plates of quartz glass and a system for implementing the process are known from DE-A1 32 26 451. In this case, a solid cylinder of quartz glass is lowered into a furnace, where it is heated to temperatures of 1,700.degree.#1,900.degree. C. to melt it. The flow of molten glass is guided into a graphite crucible. After cooling, the quartz glass obtained from the furnace can be removed from the graphite crucible as a plate. The plate thus produced is characterized by a high level of optical homogeneity. The production process, however, consumes a great deal of time and energy.
All of the processes cited above demand the production of a precursor, from which the plate must then be obtained by a series of complicated operations. DE PS 153,503 describes a process for the production of shaped products such as quartz glass plates, according to which quartz sand is spread over a substrate, possibly of quartz sand also, and then melted point by point with an electric arc. As a result, cohesive areas of vitreous quartz sand are obtained in the form of a plate. The optical quality of such plates, however, is very poor.
A process and a system are also known from U.S. Pat. No. 4,363,647, where a flat layer of quartz glass soot is deposited continuously by means of a series of hydrolysis burners onto a support body such as presintered, vitrified sand or highly pure quartz glass; the soot is then vitrified by means of a ribbon-flame burner to produce a quartz glass plate. The soot layer can be vitrified over its entire thickness. As this is done, it fuses to the support body. By means of the known process, it is therefore possible to produce TiO.sub.-- -containing compressive strain layers on glass plates. The support body can also be mechanically removed again, but this is a complicated and extremely difficult operation, especially when the layers of glass are thin. If the soot layer is not supposed to become fused to the support body, the heat must be turned off before the melting front reaches the support body. The melting front, however, is not even. Its progress depends on the local density of the soot layer and on the local heat output. If the heat is turned off before the soot layer has melted all the way through, therefore, certain areas of soot will remain unvitrified, which is unavoidable. As a result, the glass plate obtained will be uneven, although it can be removed easily from the support body.
In the patent abstract of JP-A 4-55331 (Patent Abstracts of Japan C-948, Jun. 8, 1992, Vol. 16, No. 249), the batchwise production of SiO.sub.2 glass panes is described. In this case, layers of granular quartz glass are separated from each other by layers of porous SiO.sub.2 bodies; these layers are stacked on top of each other and sintered. The separating layers are intended to prevent the quartz glass layers from fusing together and to facilitate the elimination of gas from the granular quartz glass. As a result of the weight of the layers on top, however, the layers of granular quartz glass at the bottom are compacted and therefore sinter more quickly. As a result, there is the danger that the separating layers at the bottom will sinter together with the granular quartz glass between them to form a homogeneous block before the upper layers are sintered all the way through.